Method for Monitoring the Energy Content of a Water Storage Tank System

ABSTRACT

A method of controlling a heat up of a domestic hot water tank, whereby tank temperatures are measured. The tank temperatures are fed into a control unit and a heating device is activated to heat up water in the tank when the control unit calls for heat. The method comprises a step of feeding an incoming cold-water temperature value into the control unit and generating a parameter effected by this incoming cold-water temperature. The tank temperature is measured and is related to the thermal energy content of the tank, whereby the tank temperature is measured with an integral water temperature sensor mounted to the tank, further effecting the parameter. It processes then a call for heat depending on a comparison of a set point value with the parameter generated from the tank temperature and the measured cold-water temperature.

FIELD OF THE INVENTION

The present invention relates to a method for monitoring the energy content of a water storage tank system.

It is noted that citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.

Future demand controlled grid systems need information about the energy quantity of storage systems for peak shaving and net stabilization reasons. In particular, the storage capacity of electric heated water tank systems is important for grid stabilization when considering electric energy generation from wind and solar sources.

Introducing a demand controlled water heating system makes it possible to optimize the power generation when under considering low cost power distribution and grid stability. In the future consumers can offer their water storage tank capacities to utilities to store thermal energy in times when wind and solar energy is abundant. Current state of the art is to provide an information about the temperature in a water tank using a capillary tube which contains a heat sensitive fluid in contact with the water tank. US 2009 100 199 31 A1 provides information about such a system.

DE 10 2010 047 368 B3 shows a method of controlling a water storage tank heated with an electric element. The heat up time for the water storage tank is based on information about the consumer's history of withdrawal using different supercharging rates for temperature settings.

Both methods are expensive and/or not accurate in order to supply exact information about useful energy content in the water storage tank.

DE 10 2004 018 034 B4 shows a heat pump connected to a hot water storage system with a cold-water inlet in the lower part and a hot water supply in the upper section, linked by a heat exchanger. A temperature sensor is located in the lower part of the tank. Alternatively, an impeller flow meter is located in the water pipe. The sensor or flow meter triggers operation of the heat pump if the amount of tapped water is more than a defined threshold value.

SUMMARY OF THE INVENTION

It is an objective of this invention to provide a method to reduce energy consumption of a water tank heating system. The scope of the invention is to provide accurate information about a water storage tank system.

This objective is solved by a method where a tank temperature is measured and the tank temperature is fed into a control unit. A heating element is activated to heat up warm water in the tank when the control unit calls for heat. An incoming cold-water temperature value is fed into the control unit and it generates a parameter effected by this incoming cold-water temperature. The tank temperature measured is related to the thermal energy content of the tank. The tank temperature is measured with a water temperature sensor mounted to the tank, further affecting the parameter with the tank temperature. A call for heat is processed depending on a comparison of a set point value with the parameter which is generated from the tank temperature and the measured cold-water temperature.

“The temperature sensor mounted to the tank” means that this temperature sensor could be mounted inside the tank or outside the tank. If it is mounted outside the tank the temperature sensor is especially mounted on an outer surface of the tank.

One aspect of the invention is to supply permanently exact information of the energy content of the water storage tank system to a utility in order to meet the requirements of consumer comfort and the utility to save costs and stabilize the grid.

One aspect of the invention is to calculate an exact value of thermal energy content as a function of reference mass and the generated parameter.

Another aspect of the invention is the step to measure the internal water temperature of the tank via an integral temperature sensor.

Additionally, a further aspect of the invention describes a call for heat when the exact temperature value is less than the set point value.

It is possible that the step of activating the heating device depends on the amount of the calculated exact value.

In an embodiment the step of activating the heating element follows when the amount of the calculated exact value is less than a specified value of thermal energy content.

According to a further aspect of the invention it is described to calculate the exact value of thermal energy content by multiplying the reference mass parameter by a temperature rise value.

According to this aspect the temperature rise is the difference between the set point temperature and the incoming cold-water temperature.

According to a further aspect of the invention it is described to compare the measured actual cold-water temperature signal with a previous stored cold-water temperature, to update the actual cold-water temperature signal if it is different than a previous one and to use the updated one to calculate the parameter.

According to this aspect a step of updating is followed when the actual cold-water temperature signal is lower than the previous one.

In an embodiment the method comprises the step of updating, if the actual cold-water temperature signal is different than a defined value.

According to a further aspect of the invention a step is described identifying a tapping event to detect the true cold-water temperature in an adequate duration after a tapping begins.

According to a further aspect of the invention a step is described of detecting the cold-water temperature in an adequate duration, wherein an adequate duration begins after the end of an inadequate duration.

According to a further aspect of the invention a step is described for determining the inadequate duration when the time frame is exceeded.

According to a further aspect of the invention a step is described for determining the time frame of approximately 5 to 40 seconds.

In an embodiment the adequate duration starts with the end of the inadequate duration and ends after more than 30 seconds, or at the end of the tapping event.

In an embodiment the invention comprises the step of comparison of a successive cold-water temperature with differences using the sensor temperature information and a time base.

According to a further aspect of the invention it is described to detect the tapping begin if the difference of two successive cold-water temperature values is more than 2 K during the first time base.

According to a further aspect of the invention a step is described to start a first timer 1 with an inadequate duration.

According to this aspect it follows the step of comparison an actual cold-water temperature with a previous one to calculate a difference. If the difference is less than a special parameter, a timer two is activated to measure the actual cold-water temperature information via the temperature sensor during a time base.

According to a further aspect of the invention there are described steps to detect, after second timer 2 is started, the incoming cold-water temperature frequently for a minimum duration between 5 to 30 seconds, to put each single following cold-water temperature value in a register, and compare the successive measured values. If a difference of less than a second parameter exists the measurement is valid.

It is possible to average the cold-water temperature values in the register, compare an actual averaged cold-water temperature with the former stored averaged value, and if the difference of the actual calculated value compared to the former stored value is more than 2 K, the former stored value is updated using a simple moving averaging process to avoid any jumping of values on a display.

According to a further aspect of the invention it is described to identify a tapping event with an adequate duration in order to detect the real cold-water temperature by comparison of previous cold-water temperature values with the measured actual cold-water temperature signal, storing a lowest value of cold-water temperature of the previous cold-water temperature values and the measured actual cold-water temperature signal, and using the stored lowest value to calculate the exact value of thermal energy.

According to a further aspect of the invention a step is described in which the energy needs of at least one tank is provided or sent to an energy supplier.

According to this aspect it follows the step for utilities to sort the consumers into different energy demand classes in order to stabilize the voltage or frequency of the grid.

According to a further aspect of the invention comprises a step that the water heating unit can be remotely controlled by the utility.

In an embodiment the ( ) usable hot water temperature range is between 30° C. to 50° C.

In an embodiment the lowest usable hot water temperature is close to 40° C.

In an embodiment a disinfection hot water temperature is close to 55° C., 60 C or more.

The desinfection hot water temperature is especially activated depending on an amount of tapped water in a time period.

According to a further aspect of the invention the heating element is an electric heating element.

According to a further aspect of the invention the heating device is a heat exchanger of a heat pump or the tank is connected to a heat pump or refrigerant circuit. The heat pump is activated when the electronic device or a thermostat calls for heat.

According to a further aspect of the invention the content of the water heater must be tapped or renewed in a specific time period.

According to a further aspect the time to renew or tap the content of the tank could be three days.

According to a further aspect the set temperature of the water in the tank is reduced to a lower temperature than 60° C., especially to 55° or less.

The available amount of mixed water is an important piece of information for the consumer and depends on the tank volume, mixing effect in the cold bottom of the tank below the element, and of course the expected hot water temperature and the temperature of the cold incoming water. For example, if the expected hot water temperature in the tank is 85° C. at 100-liter heated tank volume and the usable water temperature is 40° C. for 15° C. and 10° C. incoming cold-water temperature, the amount of mixed water is calculated, equation [0]:

In numbers, equation [1]:

$\begin{matrix} {{{{Mm} = \frac{{Mhw}*\left( {{Tsw} - {Tcw}} \right)}{{Tmw} - {Tcw}}}{{Mm}\mspace{14mu} {amount}\mspace{14mu} {of}\mspace{14mu} {mixed}\mspace{14mu} {water}}{{Tmw}\mspace{14mu} {Mix}\mspace{14mu} {Water}\mspace{14mu} {temperature}}{{Mhw}\mspace{14mu} {available}\mspace{14mu} {heated}\mspace{14mu} {tank}\mspace{14mu} {volume}}{Tsw}\mspace{14mu} {Expected}\mspace{14mu} {hot}\mspace{14mu} {water}\mspace{14mu} {temperature}\mspace{14mu} \left( {{set}\mspace{14mu} {point}} \right)}{{Tcw}\mspace{14mu} {Temperature}\mspace{14mu} {incoming}\mspace{14mu} {cold}\text{-}{water}}} & {{Equation}\mspace{14mu}\lbrack 0\rbrack} \\ {{{Mm}_{15} = {\frac{1001*\left( {85 - 15} \right)\left( {{^\circ}\mspace{14mu} {C.}} \right)}{40 - {15\left( {{^\circ}\mspace{14mu} {C.}} \right)}} = 2801}}{{Mm}_{10} = {\frac{1001*\left( {85 - 10} \right)\left( {{^\circ}\mspace{14mu} {C.}} \right)}{40 - {10\left( {{^\circ}\mspace{14mu} {C.}} \right)}} = 2501}}} & {{Equation}\mspace{14mu}\lbrack 1\rbrack} \end{matrix}$

In this example the amount of useful water is reduced by more than 10% if the variation of incoming cold-water is more than 5 K. A 5 K variation of the incoming cold-water temperature is a good average value if the seasonal ambient temperature between summer and winter season is taken into account.

Also, there is a variation of the annual average incoming cold-water temperature depending on the geographic location.

In another embodiment of the invention tank data is provided to utilities where a need of electric energy is calculated for heat up time.

With this data from different tank systems utilities can remotely control the tanks and define the heat up start or activation of the heating device.

Herewith a load shifting is possible on the utility's grid, especially to control the network voltage or network frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a relative inaccuracy of a mix water amount at different incoming cold-water temperatures.

FIG. 2 shows the average ground water temperature in the US by location.

FIG. 3 shows a water storage system.

FIG. 4 shows a water storage system with a tilted heating device.

FIG. 5 shows a water storage system with a flow sensor.

FIG. 6 shows a result of a feedback of different water heater devices.

DETAILED DESCRIPTION OF EMBODIMENTS

It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements which are conventional in this art. Those of ordinary skill in the art will recognize that other elements are desirable for implementing the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein.

The present invention will now be described in detail on the basis of exemplary embodiments.

For example, in the US there is a variation of the annual average incoming water temperature by location of 40° F. A calculation shows in case of 15° C. incoming water temperature and a full loaded tank the customer gets 280 l mixed water out of the tank. If the incoming cold-water temperature changes from 15° C. to 10° C. the amount of hot water produced with the same tank is 30 l less than the indicated value. For an accurate energy monitoring device, it is important to take a variation of incoming cold-water temperature into account or more if units are installed in a location where the temperature of the incoming cold-water temperature differs from the estimated value.

The following graph FIG. 1 shows the inaccuracy of the indicated mix water amount if an estimated incoming cold-water temperature of 15° C. is considered. The example is basing on a 300-l tank with an integral sensor and 60° C. hot water temperature setting.

Referring to FIG. 1 the following is cited as an example. If the incoming water temperature is 15° C. the deviation of the displayed mixed water amount is 0.

If the tank has a load status of 0.5, that means the tank is half empty, the effective available amount of usable hot water is 20% less for 8° C. cold-water and 14% less for 10° C. incoming cold-water in comparison to the displayed amount.

If the incoming cold-water temperature is 25° C. and the tank is 50% empty, the displayed amount of mixed water is 28% less than the amount which is actually available.

With this invention an energy monitoring system is created which has a higher accuracy under consideration of alternating seasonal incoming cold-water temperatures, and a standard energy monitoring device is offered for different locations from northern to southern climates. The higher accuracy is established if the incoming cold-water temperature is measured using a temperature sensor which needs to be located close to the incoming cold-water inlet tube.

The embodiment describes a system which controls the energy content of a water tank using an integral sensor and an additional sensor located close to the incoming cold-water tube.

There are several temperature sensors 8, 9, 10 mounted to the tank 1. The temperature sensor 8 situated close to the pipe where the hot water leaves the tank, an integral temperature sensor 9 which is mounted in a vertical direction of the tank 1 in order to create a thermocouple chain of a multiple sensor elements, and a bottom temperature sensor 10 situated close to the bottom incoming cold-water port. The electronic device 5 is connected to the heating device 4 to bring the heating device 4 into operation if the water needs to be heated. In this embodiment the heating device is an electric heating element. In another case it could be a heat exchanger of a heat pump or the tank 1 is connected to a heat pump or refrigerant circuit.

A safety cut-out is required to shut the heating device 4 down in case of a failed electronic control 5 or temperature sensor 8, 9, 10 failures. The safety cut-out is not shown in FIG. 3. All temperature sensors 8, 9, 10 may be mounted on the tank surface using a tape or using welded sensor sleeves which are in contact with water on well-placed spots.

The electronic device 5 has a microcontroller which controls the amount of usable hot water in the tank 1 using the equation [0]. The temperature difference between temperature sensor 8 and a temperature set point is calculated and the temperature set point is adjustable on the actuator panel 7. The energy content of the tank 1 is recorded in energy units [KWh] and the temperature difference is determined between a measured temperature and the temperature setting.

Due the fact that the thermal conductivity of the steel tank 1 surface and the thermal conductivity of the water transfer heat from the heated top section 101 above the heating device 4 to the cold bottom section 102 of the tank 1, a cold sump 103 is heated. This creates an inaccurate measurement result of the incoming water temperature. Also, depending on the incoming cold-water pipe 3 section between the water storage tank 1 and an entrance of a pipe into a building where the water tank 1 is installed more or less heat gets transferred from the ambient conditions to the cold-water, which warms up the water in this tube section 104.

FIG. 3 shows a water storage system which consists of a tank 1 with a hot water outlet pipe 2, an incoming cold-water inlet pipe 3, a heating device 4, an electronic main board 5, a display 6 and an actuator panel 7. There is a flow sensor 106 provided to measure the flow of water into the tank 1.

With this flow sensor 106 the exchange of water in the tank is measured depending on the amount of tapped water in a time period that could affect the set point temperature.

The real incoming cold-water temperature needs to be detected with the following steps which are integrated in the microprocessor software as part of the electronic device 5 including an electronic main board.

-   -   1. Identify a tapping event with an adequate duration in order         to detect the real cold-water temperature by comparison of         successive cold-water temperature values using the temperature         information of sensor 10 and a time base of 5 seconds.     -   2. Tapping begin is detected if the difference of two successive         cold-water temperature values is more than a defined         temperature, for example 2 K. Start a first timer 20.     -   3. Timer 20 compares the successive measured cold-water         temperature values with a time base of 2 seconds. If the         difference of two successive measured temperature values is less         than 1 K, start a second timer 21     -   4. Timer 21 detects the incoming cold-water temperature every 2         seconds for a minimum duration of more than 10 seconds and puts         each single following temperature value in a register. Timer 21         and 20 are running in parallel and if the comparison of         successive measured values shows a difference of more than 2 K,         both timers are stopped and the measurement is not valid.     -   5. If the measurement is valid, averaging the cold-water         temperature values takes place in the register.     -   6. Compare the actual averaged cold-water temperature with the         former stored averaged value.     -   7. If the difference of the actual calculated value compared to         the former stored value is more than 2 K overwrite the former         stored value using simple moving averaging process.     -   8. For initial start a factory temperature value of 15° C.         cold-water temperature is stored.

The remaining energy content Q_(energy) in the tank is calculated using the following law or equation [2]:

Q _(energy) =M _(m cw) *CP _(Water)*(T _(mw) −T _(cw))   Equation [2]:

-   M_(m cw) amount of mixed water based on cold-water temperature -   T_(mw) mix water temperature -   CP_(water) specific heat capacity of water -   Tcw temperature incoming cold-water

The energy content of the tank Q_(energy set point) is the energy content if the thermostat of the tank is satisfied. It is calculated with the following equation [3]:

Q _(energy set point) =M _(m setpoint) *CP _(Water)*(T _(mw) −T _(cw))   equation [3]:

-   -   M_(m) setpoint amount of mixed water based on cold-water         temperature and setpoint temperature     -   T_(mw) mix Water temperature     -   CP_(water) specific heat capacity of water     -   T_(cw) temperature incoming cold-water

The amount of energy Q_(energy reload) needed to be recharged is calculated—equation [4]:

Q _(energy reload) =Q _(energy set point) *−Q _(energy)   equation [4]:

The value of Q_(energy reload) is permanently calculated in the software of the electronic device 5.

The energy content information of the water tank is transmitted from the electronic device 5 over line 13 to a ripple control transmitter device 14 which can be connected to a structure like the internet.

For remote control the ripple control tuner 12 is connected via line 11 to the electronic device 5. Both devices 12 and 14 may be integrated with the electronic device 5. It is an advantage to deliver the energy content signal in form of a pulse pattern in a frequency range between 110 Hz-2000 Hz.

This is an advantage because it is of interest to a grid owner to sort consumers into different energy demand classes in order to be able to stabilize the voltage of the grid structure by remote control.

FIG. 6 shows the result of feedback of different water heater devices connected with a ripple control system. Due to the loading information of each single water tank 1 system, the units get categorized in terms of their individual loading demand. For example, the number of vertical lines in category [A] represent the number of consumers who need a loading capacity of less than 2.5 KWh each. Those units are transmitting their loading demand via their ripple control transmitter a 110 Hz signal to the grid owner. Categories [B], [C] and [D] are categories for other loading capacity needs.

A tank 1 volume is also calculated.

A varied incoming cold-water flow is measured with a sensor.

An incoming cold-water temperature is a value which is corresponding to the real water temperature of the cold-water source. This value could be determined by measuring the cold-water temperature by the water utility in a water treatment facility or in a main feed line of the utility. This value is transmitted to the control unit where the energy content of the water tank is calculated. Otherwise it is an embodiment of the invention, that the cold-water temperature is measured in a water line of the building where the water tank is located or in or at the water tank.

An embodiment of the sensor 10 is an at least single sensor element in close distance to a water inlet 104. It could be mounted inside the cold sump 103 or at the bottom of the tank 1, at the water inlet 105, outside or inside, or in the area of the tube section 104. In a special case where the incoming cold waterpipe 3 is situated through the top section 101 of the tank 1, the sensor could be applied in the area of the incoming cold-water pipe 3 or other water inlet.

An embodiment of the integral sensor 9 is a chain of thermocouples especially in line, vertically mounted, a wound wire with a defined length consisting of material with NTC or PTC characteristics, a layer with a printed sensor or a multiple printed sensor chain or at least a couple of sensors in a series or parallel circuit. The integral sensor is located inside or outside the tank 1.

Data acquisition is realized by wire or wireless communication between the temperature sensor 8, 9, 10 and the electronic device 5 or other control unit.

The temperature sensor 10 should be installed at a place of the real cold-water temperature. This place is in another embodiment of the invention one or more representative locations where the temperature sensors 10 are located. If the temperature sensors 8, 9, 10 are mounted outside of the water tank 1 at a main water pipe or a house water pipe, it is an advantage when the temperature sensors 8, 9, 10 are connected to a wireless communication transmitter to send the measured incoming cold-water temperature to the control unit.

A further embodiment of the invention is to receive local cold-water temperature values from a utility or other source and actually store them in the electronic device 5 and to use them as cold-water temperatures instead of or in addition to a measured temperature value from the cold water incoming sensor 10. Otherwise a cold-water temperature value can be entered to the electronic device 5 by the user manually or via verbal command.

It is possible to retrofit existing water heating systems with this method.

While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the inventions as defined in the following claims.

It is noted that citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention. 

1. A method of controlling a heat up of a hot water tank having a control unit, the method comprising: measuring a tank temperature; feeding the tank temperature into a control unit; activating a heating device to heat up water in the tank when the control unit calls for heat; feeding an incoming cold-water temperature value into the control unit; generating a parameter effected by the incoming cold-water temperature; measuring the tank temperature which is related to the thermal energy content of the tank, where the tank temperature is measured with a sensor mounted to the tank; modifying the generated parameter based on the tank temperature; processing a call for heat depending on a comparison of a set point value with the modified parameter.
 2. The method according to claim 1, further comprising: calculating an exact value of thermal energy content as a function of reference mass and the modified parameter.
 3. The method according to claim 2,further comprising: calling for heat when the exact value is less than the set point value.
 4. The method according to claim 1; wherein the tank temperature is measured with an integral temperature sensor.
 5. The method according to claim 2, further comprising: activating the heating device depending on the amount of the calculated exact value.
 6. The method according to claim 5; wherein the heating element is activated when the amount of the calculated exact value is less than a specified value of thermal energy content.
 7. The method according to claim 1, further comprising: calculating the exact value of thermal energy content by multiplying the reference mass parameter by a temperature rise value.
 8. The method according to claim 7; wherein the temperature rise is the difference between the set point temperature and the incoming cold-water temperature.
 9. The method according to claim 1, further comprising: comparing a measured actual cold-water temperature signal with a previous stored cold-water temperature; updating the stored cold-water temperature value if the measured actual cold-water temperature signal is different from the previous stored cold-water temperature; and utilizing the updated stored cold-water temperature value to calculate the parameter.
 10. The method according to claim 9; wherein the stored cold-water temperature value is updated when the actual cold-water temperature signal is lower than the previous stored cold-water temperature.
 11. The method according to claim 9; wherein the stored cold-water temperature value is updated if the actual cold-water temperature signal is different from a defined value.
 12. The method according to claim 1, further comprising: identifying a tapping event to detect a true cold-water temperature in an adequate duration after a tapping begin.
 13. The method according to claim 12, further comprising: detecting the cold-water temperature in an adequate duration, which adequate duration begins after the end of an inadequate duration.
 14. The method according to claim 13, further comprising: determining the inadequate duration when a time frame is exceeded.
 15. The method according to claim 14; wherein the time frame is approximately 20 seconds.
 16. The method according to claim 13; wherein the adequate duration starts with the end of the inadequate duration and ends after more than 30 seconds or at or after the end of the tapping event.
 17. The method according to claim 13, further comprising: comparing successive cold-water temperature measurements with differences using the temperature information of sensors and a time base.
 18. The method according to claim 17, further comprising: detecting the tapping begin if a difference of two successive cold-water temperature values is more than 2 K of the time base.
 19. The method according to claim 18, further comprising: starting a first timer with an inadequate duration.
 20. The method according to claim 19, further comprising: comparing an actual cold-water temperature with a previous one to calculate a difference and, if the difference is less than a special parameter, activating a second timer to measure the actual cold-water temperature information of a sensor during a time base.
 21. The method according to claim 20, further comprising: after the second timer is started to detect the incoming cold-water temperature frequently for a minimum duration of more than 10 seconds, putting each single resulting cold-water temperature value in a register, and comparing the successive measured values; and determining that the measurement is valid if a difference of less than a second parameter exists.
 22. The method according to claim 21, further comprising: averaging the cold-water temperature values in the register, comparing an actual averaged cold-water temperature with the former stored averaged value; and if the difference of the actual calculated value compared to the former stored value is more than 2 K, updating the former stored value using a simple moving averaging process.
 23. The method according to claim 1, further comprising: identifying a tapping event with an adequate duration in order to detect the real cold-water temperature by comparison of previous cold-water temperature values with the measured actual cold-water temperature signal; storing a lowest value of cold-water temperature of the previous cold-water temperature values and the measured actual cold-water temperature signal; and utilizing the stored lowest value to calculate the exact value of thermal energy.
 24. The method according to the claim 23, comprising the step in which the energy needs of at least one tank 1 are provided to an energy supplier or sent to a utility.
 25. The method according to claim 1, further comprising: a utility company sorting consumers into different energy demand classes in order to stabilize a voltage, a frequency, or both, of an electric grid.
 26. The method according to claim 24; wherein the heating unit is configured to be remotely controlled by the utility company.
 27. The method according to claim 1; wherein the sensor mounted outside the tank.
 28. The method according to claim 1; wherein the sensor mounted inside the tank. 